What exactly is "image quality"? How would you compare it objectively?
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mattdmJan 9 '11 at 0:38

There's several factors that I could see this including. They include the amount of time it takes to take a picture, the amount of noise on the image and other qualifiers, and probably some others as well.
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PearsonArtPhotoJan 9 '11 at 1:47

Noise, real sensitivity (time/aperture/ISO for fixed light), resolution, aberrations are factual comparable for compact cameras (as some of these parameters are for lenses for SLR).
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LeonidasApr 27 '11 at 0:20

Just look at sample photos from the cameras you are interested in - if you can't tell which one has the best 'technical image quality' then does it matter? Are you going to get some satisfaction from looking at a photo you've taken, knowing it has better image quality than someone else's photo, even if theirs is a better picture?
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Steve IvesJul 3 at 6:58

Note that the reviews at dpreview.com not only contains sample images, but also test images of standard scenes as well as comparisons between how different cameras handles the test shots.
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GuffaJan 9 '11 at 1:48

DxO Mark provide objective data on the output of camera and cameras in combination with lenses.

The data comes from the testing they do on cameras and lenses for their RAW developing software "DxO Optics" so it should be fairly unbiased. I don't know enough about the technical aspects of digital photography to comment on the validity of their methodology.

While DXOMark is a nice metric that has some measurement value, it encompasses very little of what constitutes image quality.
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ItaiJan 9 '11 at 2:28

1

If there were some way by which I could vote +100 to what Itai says here, I would!
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mattdmJan 9 '11 at 16:37

1

Interesting. I expected there to be objections, but I thought they would relate to how the data was collected, not the metrics themselves. Surely, for example, the signal-to-noise for a given ISO value has a strong correlation to image quality in environments where you're forced to use higher ISOs? Similarly, anyone who shoots under (for example) bright stage lights would find a camera with a large dynamic range would improve the image quality in their highlights and/or shadows?
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Scott CarrollJan 9 '11 at 17:43

To my knowledge DxO Mark conduct the most comprehensive, thorough, careful and exact set of photographic measurements that are published on the Internet. My main criticism is that they could use better methods for measuring lens resolution. For example use a microscope to examine the aerial image or better still, use Hartmann-Shack wavefront analysis.
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labnutApr 28 '11 at 8:39

@mattdm, do you not believe that, in principle, careful measurements bring greater knowledge and insights to many subjects, including technical image quality?
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labnutApr 28 '11 at 8:42

It probably doesn't matter. Taking great looking photos has much more to do with the photographer than the camera. A good photographer can take great photos with any camera.

I suggest you decide what kind of camera to get based on the kind of shooting you do or intend to do, then visit snapsort.com to find a good model in that class. If you're going to take a lot of low light photos, get something with a fast lens; if you'll be shooting over a long distance, get something with a good zoom, and so on.

However, I agree with the suggestion to visit dpreview.com. They have extremely comprehensive reviews, which includes image quality and comparisons against other cameras.

While I agree with the basic sentiment, I think it's worth noting that different cameras are better for different purposes. For an extreme example, consider a Sinar -- undeniably a good camera, but I sure wouldn't want to use it to shoot a basketball game!
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Jerry CoffinJan 9 '11 at 3:08

This is exactly what I suggested: decide which broad category of camera he wants, then use SnapSort to recommend a specific model.
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ieureJan 9 '11 at 6:10

3

It's a nice sentiment but you've clearly not tried to use the camera on my old cell phone. It was truly terrible. In the hands of the greatest photographer alive you might get some interesting shots, but I think the same photographer would get even better photos (and a lot more of them) with a high quality camera and lens. With this in mind, why not strive to get the best camera in addition to being a good photographer?
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Matt GrumJan 9 '11 at 12:29

That's irrelevant, since he isn't going to buy the camera on your old cell phone. While I'm not disputing that there are quality differences between cameras, I think the image quality between newly purchased cameras isn't drastic enough to be the single issue on which a purchase hinges.
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ieureJan 9 '11 at 22:54

It often does matter a great deal and anyway artistic quality deserves to be displayed with the highest technical quality. I take/print many large panorama photos. Here I find exceptional sharpness and flatness of field really does matter. I observe people as they view my prints. They first step back to take in the print as a whole and then, without exception, step closer to view the details close up. And then they react with pleasurable surprise to see the fine detail faithfully preserved.
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labnutApr 27 '11 at 10:15

First off, comparing images on the Internet is a misleading business since the resolution is usually too low to show up differences. Images from my old 2.1 Mp Canon P&S look not much inferior to my present 15 Mp DSLR when displayed in a normal web page.

However DPReview.com have, very fortunately for us, had a policy over many years of taking photos of the same resolution chart, under the same conditions and storing the original image for download. It is a marvelous resource. There you can find resolution images going back many years for all the major camera models.

Technical image quality has several dimensions and all need to be measured if you want a complete result. For many people though, resolution and chromatic aberration are the most important results. Luckily these things are easy to measure if we use the DPReview.com resolution image library.

Imagine we want to compare the following cameras (selected for no particular reason)
You can find the converted raw images under the Resolution heading of the camera reviews:

Open each image in Quickmtf. You will see a number of 5 degree slanted edges at different points in the chart. Choose one of interest to you.

Using the cursor tool, draw a rectangle to enclose part of the slanted edge.

A graph will open up showing the MTF curve. Now you can read off things like line pairs per picture height.

In the tool bar click on the buttons for Edge Spread and Line Spread. You will be shown the edge spread and line spread functions. Here you can read off the edge blur and the chromatic aberration.

Results

Horizontal edge, 15% from the center.
Edge blur in pixel (chromatic aberration in pixel).
For reference - 1.27 pixel is the best attainable result.

Nikon D7000........2.5 px (0.8 px)

Nikon D5100........2.6 px (0.6 px)

Nikon D3100........2.1 px (0.7 px)

Canon 600D.........2.4 px (0.6 px)

Panasonic DMC-G2...2.0 px (0.2 px)

I show edge blur because it is immediately understandable to most people. Reading MTF graphs is challenging. The important thing to note is the ringing in the graphs for Canon and Panasonic. This indicates that the images have been sharpened more than the images for Nikon, which distorts the results. Ideally the images should not have been sharpened, but we have to work with what is available.

This tool lets you compare reference images from digital cameras side-by-side. These images are JPEGs straight from the camera, and were taken under carefully-controlled conditions, to provide valid comparisons of camera capabilities in actual shooting situations. You can also download the images (using your browser's "save image as" function) and output them on your own printer, to see how the cameras involved will perform in your application.